1. Field of the Invention
This invention relates generally to programmable manipulator apparatus, commonly known as robots, which enable the positioning, in at least two dimensions, of a workpiece altering tool, and further to methods of operating these apparatus. In one specific embodiment, the present invention relates to a manipulator system specifically adapted for welding.
2. Brief Description of the Background Art
In the past a number of operations required to transform a workpiece from a first state to a second or usable state have been done in a manual or semi-automatic fashion. In manual operations, the workpiece is totally transformed to its usable state by worker manipulations. In the semi-automatic mode a tool which holds the workpiece and simultaneously operates under selective control by an operator is utilized. Of course the success of either the manual or semi-automatic operations is limited, to varying degrees, by the operator's own capability.
In relatively recent times automatic machines have become available for workpiece operations. For example, a number of commercial robots are available which are capable of performing various operations on a workpiece. Devices of this kind are known for painting, welding, machining, part assembly, article transfer and the like. However, many of these commercial devices are limited to specific motions or specific operations on very limited types of workpieces. For example, a given welding robot may be adaptable to only certain types of workpieces. If a deviation or irregularity occurs in the workpiece, the robot is unable to deal with the unexpected situation. Thus a conventional manipulator apparatus is usually subject to very poor flexibility in operation and specifically to a very limited ability to adapt to changed circumstances.
The ultimate aim for many applications in the field of robotics is to develop a device capable not only of performing operations in an automated fashion, but moreover, and perhaps more importantly, capable of, at least to a limited degree, programming subsequent operations on its own. Despite the many advantages which have been achieved through the development of automated devices, it is recognized that much of the programming operation, particularly in the field of numerical controls, is extremely tedious and very time consuming. If this programming is not performed accurately the overall quality of the operation may be less acceptable than that achieved with the conventional manual or semi-automatic processing.
So called "self-programming" devices have been known in the past. For example, in U.S. Pat. No. 4,258,425 to Ramsey, et al. a self-programmed mechanical working apparatus is disclosed. This patent illustrates a welding robot capable of progressing along a seam to be welded on its own and memorizing point coordinate/slope data along the seam, using a seam contacting probe, to enable subsequent performance in a playback mode of the programmed welding operation. The memorized points are recorded in a periodic fashion without any decision being made as to which points should be programmed. In addition a third degree polynomial or spline function is used to interpolate between or to connect the various memorized points. The probe is removed or retracted in the playback mode.
While relatively complex mathematical interpolation techniques may be capable of achieving an extremely close fit to a desired weld path, the generation of these equations could have a number of disadvantages. The most important of these disadvantages is that a great deal of computing power may be tied up in generating the polynomial calculations. Furthermore, a great deal of memory may be tied up in recording the relatively large amount of data necessary to implement these techniques.
It has also been suggested in the prior art to utilize in a playback mode only certain significant points along a path to be recorded. In U.S. Pat. No. 4,130,873 to Fioretta, et al., an automatic machine with an articulated mechanical arm uses a straight line interpolation technique to connect various "significant" points along a path to be followed. However, the apparatus records all of the data points on magnetic tape. This data is subsequently processed to derive the significant points. Thus, the apparatus disclosed may be capable of recording a desired path of movement along a workpiece in a rough kind of way. The device may thereafter be capable of following the work path to what may amount to an acceptable degree of preciseness. However, it would appear that it would not be possible to closely follow a precise path along a workpiece using this arrangement and that a significant amount of data processing would be necessary.
It may be observed that in a variety of fields in which programmed manipulators are utilized it is desirable to repetitively and identically process a number of successive workpieces. After an initial programming or path learning mode it would be desirable to thereafter consecutively operate on workpieces in a repetitive fashion. One problem that commonly arises is that subsequent workpieces are limited by various manufacturing parameters so that they may not all be precisely identical. These irregularities in specific subsequent workpieces may arise from a number of factors including out-of-tolerance parts, manufacturing defects, material defects, and distortions which occur randomly during the manufacturing process.
When the programmed apparatus attempts to follow a workpiece seam to be welded and it encounters distortions, conventional apparatus may be incapable of adapting to such unexpected conditions. The program "tells" the apparatus to follow a certain pattern of movement and yet the workpiece itself may prevent the apparatus from doing so. The usual result is shutdown of the apparatus, but sometimes damage to the machine and the workpiece may result as well.
This problem was recognized in the Ramsey patent described above. However, in response to an out-of-tolerance condition, the device disclosed in the Ramsey patent merely provides an indication to the operator and shuts down. Then operator intervention is required so that truly automated operation is no longer possible.
The inventors of the present invention have appreciated that it would be highly desirable to provide a programmed manipulator, adaptable to unexpected conditions during the performance of an automated operation, that requires a minimum of computational power and data storage capacity. However such a device must also be capable of providing the necessary degree of precision of repetitive movement. A review of the prior art would likely lead one skilled in the art to believe that these goals are inconsistent with one another or even mutually exclusive.